if we look at the end or start of the line we see a square. eg:
if we put our camera in side the line we see that it looks like an x. eg:
here is a picture with my camera inside the default pawn showing the x and the square at the end of the line.
In conclusion our camera is seeing either the end of the line or the start of the line.
I think the entire cross section of the line is a square (it is probably a long thin rectangle, but with GPUs, it’s all triangles, so each rectangle is split into two triangles), but when the camera is too close or inside the line, the renderer mis-renders the triangles (or has trouble deciding which to render and which to cull) that make up the long, thin rectangle.
I think you are right look what happens with the line when it I change view mode to wireframe.
It looks like a rectangle.